1. Field of the Invention
The present invention relates generally to a three-phase thermal protector having two pairs of contacts, a single thermally responsive element for opening and closing the two pairs of contacts, and an operative temperature calibrating mechanism for calibrating an operative temperature of the thermally responsive element, and more particularly to such a three-phase thermal protector suitable for opening and closing the neutral of wye-connected phase windings of a three-phase induction motor.
2. Description of the Prior Art
Conventional three-phase thermal protectors of the above-described type are disclosed by Japanese Published Utility Model Application No. 31-5747 entitled "DISH-SHAPED BIMETAL RELAY WITH THREE PAIRS OF CONTACTS" or Japanese Published Patent Application No. 46-34532 entitled "THERMOSTAT SWITCH." Six contacts, that is, three movable contacts and three fixed contacts are employed in the above-mentioned protectors. Such a number of contacts entails an economic problem that the manufacturing cost of the protector is increased.
The inventors have disclosed, in Japanese Laid-open Patent Application No. 57-34623, a thermal protector for three-phase induction motors, in which the number of the contacts is reduced to four, that is, two movable contacts and two fixed contacts. However, this thermal protector is the type that an operative temperature of a snap-acting thermally responsive element comprising bimetallic or trimetallic plates is calibrated by deforming a hermetically sealed housing of the protector. Accordingly, the calibration of the operative temperature of the thermally responsive element relies upon a method of adjusting the contact pressure between the contacts to a predetermined value at the room temperature after the enclosing of a movable mechanism in the housing. This method of calibrating the operative temperature results in a disadvantage that when the difference of operation time periods between the two pairs of contacts exceeds the tolerance owing to the difference of contact gaps between the respective pair of contacts, an effective measure cannot be taken to visually check or correct the difference of the operation time periods between the respective pairs of contacts.